Failure detection apparatus for an internal combustion engine

ABSTRACT

A failure detection apparatus for an internal combustion engine includes failure detecting means (S 22 -S 30 ) for detecting abnormality of fresh air quantity detecting means (air flow sensor) based on the result of comparison between a fresh air quantity detected by the fresh air quantity detecting means (air flow sensor) and a fresh air quantity reference value set by fresh air quantity reference value setting means (S 20 ), and exhaust flow rate adjusting means. The fresh air quantity reference value setting means sets the reference value (S 16 ) in accordance with not only the operating state (engine speed N e , fuel injection quantity Q f , etc.) of the engine but also a target adjustment amount (target throttle valve opening) set for the exhaust flow rate adjusting means by target adjustment setting means (S 12 , S 14 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a failure detection apparatus for aninternal combustion engine, and more particularly, to techniquesenabling reliable detection of abnormality in an air flow sensor.

2. Description of the Related Art

Recently, in order to prevent harmful exhaust gas from being emittedfrom an engine mounted on a motor vehicle, various control means areused to improve exhaust gas characteristics. These control means operatebased on information supplied from various sensors and the like, toimprove the exhaust gas characteristics.

If any of the sensors etc. falls, however, the exhaust gascharacteristics may possibly be deteriorated, and accordingly, there hasbeen a demand for reliable detection of failure of such sensors etc.Recently, vehicles equipped with an on-board diagnostic system (OBDetc.) have been developed and put to practical use, with a view tofurther improving the exhaust gas characteristics.

Information from the various sensors etc., especially, information froman air flow sensor (AFS) is used for many purposes including the controlof an after-treatment device, EGR or exhaust gas recirculation, etc.,and failure of the air flow sensor greatly affects the exhaust gascharacteristics. Accordingly, diagnosis of the air flow sensor is ofespecial importance.

Thus, an apparatus has been proposed in which, when the rotation speedof the engine is lower than or equal to a predetermined value and at thesame time an intake air quantity detected by the air flow sensor ishigher than or equal to a predetermined value, for example, the air flowsensor is judged to be abnormal (see Japanese Patent ApplicationPublication No. H10-018897, for example).

In such diagnosis of the air flow sensor, failure of the sensor isusually determined by comparing a reference value set in advance basedon operating conditions, such as engine speed, fuel injection quantity,engine torque, throttle opening (throttle valve opening), manifold airpressure, etc., with an output value from the air flow sensor.

However, in the case where the flow rate of exhaust gas is adjusted byan exhaust flow control valve arranged in the intake or exhaust systemto promote purification of the exhaust gas or where EGR gas isintroduced into the intake system while the exhaust flow rate isadjusted, the quantity of fresh air varies depending on the opening ofthe exhaust flow control valve or EGR valve, giving rise to a problemthat failure of the air flow sensor cannot be determined with accuracy.

To solve the problem, the diagnosis of the air flow sensor may besuspended while the opening of the exhaust flow control valve or EGRvalve varies, for example. In the apparatus disclosed in theaforementioned publication, for example, the diagnosis of the air flowsensor as to abnormality is prohibited when the engine is in a coldstate in which the engine is supplied with bypass air.

Such suspension of the diagnosis is, however, not desirable because itleads to a substantial reduction in the period for diagnosing the airflow sensor.

SUMMARY OF THE INVENTION

The present invention was created to solve the above problems, and anobject thereof is to provide a failure detection apparatus for aninternal combustion engine which is capable of reliable detection ofabnormality in an air flow sensor irrespective of exhaust flow ratecontrol.

To achieve the object, a failure detection apparatus according to thepresent invention comprises: fresh air quantity detecting means arrangedin an intake system of an internal combustion engine, for detecting aquantity of fresh air introduced into a combustion chamber of theengine; fresh air quantity reference value setting means for setting areference value for the fresh air quantity in accordance with anoperating state of the engine; failure detecting means for detectingabnormality of the fresh air quantity detecting means, based on a resultof comparison between the fresh air quantity detected by the fresh airquantity detecting means and the reference value set by the fresh airquantity reference value setting means; exhaust flow rate adjustingmeans arranged in at least one of the intake system and exhaust systemof the engine, for adjusting an exhaust flow rate; target adjustmentsetting means for setting a target adjustment amount for the exhaustflow rate adjusting means in accordance with the operating state of theengine such that an air-fuel ratio or excess air ratio of the exhaustsystem becomes equal to a predetermined value corresponding to theoperating state; and exhaust flow rate control means for controlling theexhaust flow rate adjusting means in accordance with the targetadjustment amount set by the target adjustment setting means, whereinthe fresh air quantity reference value setting means sets the referencevalue in accordance with not only the operating state of the engine butalso the target adjustment amount set for the exhaust flow rateadjusting means by the target adjustment setting means.

Thus, the reference value for the fresh air quantity is set by the freshair quantity reference value setting means in accordance with theoperating state (engine speed, fuel injection quantity engine torque,throttle opening, manifold air pressure, etc.) of the engine, and basedon the result of comparison between the thus-set reference value and thefresh air quantity detected by the fresh air quantity detecting means,abnormality or failure of the fresh air quantity detecting means (airflow sensor) is detected. In this case, the fresh air quantity referencevalue setting means sets the reference value in accordance with not onlythe operating state of the engine but also the target adjustment amountset for the exhaust flow rate adjusting means by the target adjustmentsetting means.

Accordingly, the reference value for the fresh air quantity can be settaking account of the target adjustment amount, that is, the amount ofexhaust flow rate adjusted by the exhaust flow rate adjusting means.Proper and reliable diagnosis of the fresh air quantity detecting meanscan therefore be carried out, irrespective of the adjustment of exhaustflow rate for promoting exhaust gas purification, whereby reliability ofthe fresh air quantity detecting means can be enhanced, making itpossible to further improve the exhaust gas characteristics.

In the failure detection apparatus according to the present invention,the exhaust flow rate adjusting means may include at least one of anintake throttle valve arranged in the intake system for adjusting thefresh air quantity and an exhaust throttle valve arranged in the exhaustsystem for directly adjusting the exhaust flow rate. The fresh airquantity reference value setting means sets the reference value inaccordance with not only the operating state of the engine but also atarget valve opening set for said at least one of the intake and exhaustthrottle valves by the target adjustment setting means.

Thus, the fresh air quantity reference value setting means sets thereference value in accordance with not only the operating state of theengine but also the target valve opening set for the intake and exhaustthrottle valves by the target adjustment setting means.

Accordingly, the reference value for the fresh air quantity can be settaking account of the target valve opening, that is, the amount ofexhaust flow rate adjusted by the intake and exhaust throttle valves.Proper and reliable diagnosis of the fresh air quantity detecting meanscan therefore be carried out, irrespective of the adjustment of exhaustflow rate for promoting exhaust gas purification, whereby reliability ofthe fresh air quantity detecting means can be enhanced.

The failure detection apparatus of the present invention may furthercomprise exhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, wherein the fresh air quantity reference valuesetting means corrects the target adjustment amount for the exhaust flowrate adjusting means, based on a difference between the air-fuel ratioor excess air ratio of the exhaust system, detected by the exhaustconcentration detecting means, and the predetermined value, and sets thereference value based on the corrected target adjustment amount.

Specifically, the target adjustment amount for the exhaust flow rateadjusting means is a command value which is set in accordance with theoperating state (engine speed, fuel injection quantity, etc.) of theengine such that the air-fuel ratio or excess air ratio of the exhaustsystem becomes equal to the predetermined value corresponding to theoperating state, and the target adjustment amount may possibly bedifferent from an actual adjustment amount. Accordingly, the targetadjustment amount for the exhaust flow rate adjusting means is correctedbased on the difference between the air-fuel ratio or excess air ratioof the exhaust system, detected by the exhaust concentration detectingmeans, and the predetermined value, and the reference value is set basedon the corrected target adjustment amount.

Consequently, the reference value for the fresh air quantity can be setto a proper value matching the actual adjustment amount for the exhaustflow rate adjusting means, and the fresh air quantity detecting meanscan be diagnosed with higher accuracy during the adjustment of theexhaust flow rate, whereby the reliability of the fresh air quantitydetecting means can be further improved.

The failure detection apparatus of the present invention may furthercomprise exhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, wherein the exhaust flow rate control meanscorrects an amount of adjustment by the exhaust flow rate adjustingmeans such that the air-fuel ratio or excess air ratio of the exhaustsystem, detected by the exhaust concentration detecting means, coincideswith the predetermined value.

As mentioned above, the target adjustment amount for the exhaust flowrate adjusting means may possibly be different from an actual adjustmentamount, and therefore, the amount of adjustment by the exhaust flow rateadjusting means is corrected such that the air-fuel ratio or excess airratio of the exhaust system, detected by the exhaust concentrationdetecting means, coincides with the predetermined value.

Accordingly, the reference value for the fresh air quantity can be setto a proper value matching the actual amount of adjustment by theexhaust flow rate adjusting means, whereby the fresh air quantitydetecting means can be diagnosed with higher accuracy during theadjustment of the exhaust flow rate, making it possible to furtherimprove the reliability of the fresh air quantity detecting means.

The failure detection apparatus of the present invention may furthercomprise exhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, wherein, when the air-fuel ratio or excess airratio of the exhaust system, detected by the exhaust concentrationdetecting means, is different from the predetermined value, the freshair quantity reference value setting means suspends the control of theexhaust flow rate adjusting means by the exhaust flow rate control meansand sets the reference value based solely on the operating state of theengine.

The target adjustment amount for the exhaust flow rate adjusting meansmay possibly be different from an actual adjustment amount, as mentionedabove, and therefore, when the air-fuel ratio or excess air ratio of theexhaust system, detected by the exhaust concentration detecting means,is different from the predetermined value, the control of the exhaustflow rate adjusting means is suspended.

In this case, the reference value for the fresh air quantity is setbased solely on the operating state of the engine, to enhance theaccuracy of diagnosis of the fresh air quantity detecting means withoutreducing the occasion for diagnosis, whereby the reliability of thefresh air quantity detecting means can be further improved.

According to another aspect of the present invention, a failuredetection apparatus comprises: fresh air quantity detecting meansarranged in an intake system of an internal combustion engine, fordetecting a quantity of fresh air introduced into a combustion chamberof the engine; fresh air quantity reference value setting means forsetting a reference value for the fresh air quantity in accordance withan operating state of the engine; failure detecting means for detectingabnormality of the fresh air quantity detecting means, based on a resultof comparison between the fresh air quantity detected by the fresh airquantity detecting means and the reference value set by the fresh airquantity reference value setting means; exhaust flow rate adjustingmeans arranged in one of the intake system and exhaust system of theengine, for adjusting an exhaust flow rate; target adjustment settingmeans for setting a target adjustment amount for the exhaust flow rateadjusting means in accordance with the operating state of the enginesuch that an air-fuel ratio or excess air ratio of the exhaust systembecomes equal to a predetermined value corresponding to the operatingstate; exhaust flow rate control means for controlling the exhaust flowrate adjusting means in accordance with the target adjustment amount setby the target adjustment setting means; an EGR passage for allowing partof exhaust gas to be recirculated from the exhaust system of the engineto the intake system as EGR gas; an EGR valve inserted in the EGRpassage, for controlling a quantity of the EGR gas by varying an openingthereof; target opening setting means for setting a target EGR valveopening for the EGR valve in accordance with the operating state of theengine such that the air-fuel ratio or excess air ratio of the exhaustsystem becomes equal to the predetermined value; and EGR valve controlmeans for controlling the EGR valve in accordance with the target EGRvalve opening set by the target opening setting means, wherein the freshair quantity reference value setting means sets the reference value inaccordance with not only the operating state of the engine but also thetarget adjustment amount set for the exhaust flow rate adjusting meansby the target adjustment setting means and the target EGR valve openingset by the target opening setting means.

Thus, the reference value for the fresh air quantity is set by the freshair quantity reference value setting means in accordance with theoperating state (engine speed, fuel injection quantity, engine torque,throttle opening., manifold air pressure, etc.) of the engine, and basedon the result of comparison between the thus-set reference value and thefresh air quantity detected by the fresh air quantity detecting means,abnormality or failure of the fresh air quantity detecting means (airflow sensor) is detected. In this case, the fresh air quantity referencevalue setting means sets the reference value in accordance with not onlythe operating state of the engine but also the target adjustment amountset for the exhaust flow rate adjusting means by the target adjustmentsetting means and the target EGR valve opening set by the target openingsetting means.

Accordingly, the reference value for the fresh air quantity can be settaking account of the target adjustment amount, that is, the amount ofexhaust flow rate adjusted by the exhaust flow rate adjusting means, andthe target EGR valve opening, that is, the EGR gas quantity. Proper andreliable diagnosis of the fresh air quantity detecting means cantherefore be carried out, irrespective of the adjustment of exhaust flowrate or the introduction of EGR gas for promoting exhaust gaspurification, whereby reliability of the fresh air quantity detectingmeans can be enhanced, making it possible to further improve the exhaustgas characteristics.

In the failure detection apparatus of the present invention, the exhaustflow rate adjusting means may include at least one of an intake throttlevalve arranged in the intake system for adjusting the fresh air quantityand an exhaust throttle valve arranged in the exhaust system fordirectly adjusting the exhaust flow rate. The fresh air quantityreference value setting means sets the reference value in accordancewith not only the operating state of the engine but also a target valveopening set for said at least one of the intake and exhaust throttlevalves by the target adjustment setting means and the target EGR valveopening set by the target opening setting means.

Thus, the fresh air quantity reference value setting means sets thereference value in accordance with not only the operating state of theengine but also the target valve opening set for the intake and exhaustthrottle valves by the target adjustment setting means and the targetEGR valve opening set by the target opening setting means.

Accordingly, the reference value for the fresh air quantity can be settaking account of the target valve opening, that is, the amount ofexhaust flow rate adjusted by the intake and exhaust throttle valves, aswell as the target EGR valve opening, that is, the EGR gas quantity.Proper and reliable diagnosis of the fresh air quantity detecting meanscan therefore be carried out, irrespective of the adjustment of exhaustflow rate or the introduction of EGR gas for promoting exhaust gaspurification, whereby reliability of the fresh air quantity detectingmeans can be enhanced.

The failure detection apparatus of the present invention may furthercomprise exhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, wherein the fresh air quantity reference valuesetting means corrects at least one of the target adjustment amount andthe target EGR valve opening based on a difference between the air-fuelratio or excess air ratio of the exhaust system, detected by the exhaustconcentration detecting means, and the predetermined value, and sets thereference value based on said at least one of the corrected targetadjustment amount and the corrected target EGR valve opening.

Specifically, the target adjustment amount for the exhaust flow rateadjusting means or the target EGR valve opening for the EGR valve is acommand value which is set in accordance with the operating state(engine speed, fuel injection quantity, etc.) of the engine such thatthe air-fuel ratio or excess air ratio of the exhaust system becomesequal to the predetermined value corresponding to the operating state,and may possibly be different from an actual adjustment amount or actualEGR valve opening. Thus, the target adjustment amount for the exhaustflow rate adjusting means and the target EGR valve opening are correctedbased on the difference between the air-fuel ratio or excess air ratioof the exhaust system, detected by the exhaust concentration detectingmeans, and the predetermined value, and the reference value is set basedon the corrected target adjustment amount and the corrected target EGRvalve opening.

Accordingly, the reference value for the fresh air quantity can be setto a proper value matching the actual adjustment amount of the exhaustflow rate adjusting means and the actual EGR valve opening, and thus thefresh air quantity detecting means can be diagnosed with higheraccuracy, further improving the reliability of the fresh air quantitydetecting means.

The failure detection apparatus of the present invention may furthercomprise exhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, wherein the exhaust flow rate control meanscorrects an amount of adjustment by the exhaust flow rate adjustingmeans such that the air-fuel ratio or excess air ratio of the exhaustsystem, detected by the exhaust concentration detecting means, coincideswith the predetermined value, and the EGR valve control means correctsthe opening of the EGR valve such that the air-fuel ratio or excess airratio of the exhaust system, detected by the exhaust concentrationdetecting means, coincides with the predetermined value.

As mentioned above, the target adjustment amount for the exhaust flowrate adjusting means and the target EGR valve opening for the EGR valvemay possibly be different from actual adjustment amount and EGR valveopening, respectively. Thus, the amount of adjustment by the exhaustflow rate adjusting means and the EGR valve opening are corrected suchthat the air-fuel ratio or excess air ratio of the exhaust system,detected by the exhaust concentration detecting means, coincides withthe predetermined value.

Accordingly, the reference value for the fresh air quantity can be setto a proper value matching the actual amount of adjustment by theexhaust flow rate adjusting means and the actual EGR valve opening, andthus the fresh air quantity detecting means can be diagnosed with higheraccuracy, further improving the reliability of the fresh air quantitydetecting means.

The failure detection apparatus of the present invention may furthercomprise exhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, wherein, when the air-fuel ratio or excess airratio of the exhaust system, detected by the exhaust concentrationdetecting means, is different from the predetermined value, the freshair quantity reference value setting means suspends the control of theexhaust flow rate adjusting means by the exhaust flow rate control meansas well as the control of the EGR valve by the EGR valve control meansand sets the reference value based solely on the operating state of theengine.

The target adjustment amount for the exhaust flow rate adjusting meansand the target EGR valve opening for the EGR valve may possibly bedifferent from actual adjustment amount and EGR valve opening,respectively. Thus, when the air-fuel ratio or excess air ratio of theexhaust system, detected by the exhaust concentration detecting means,is different from the predetermined value, the control of the exhaustflow rate adjusting means and the control of the EGR valve aresuspended.

In this case, the reference value for the fresh air quantity is setbased solely on the operating state of the engine, to enhance theaccuracy of diagnosis of the fresh air quantity detecting means withoutreducing the occasion for diagnosis, whereby the reliability of thefresh air quantity detecting means can be further improved.

A further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specific example,while indicating preferred embodiments of the invention, are given byway of illustration only, since various changes and modifications withinthe spirit and scope of the invention will become apparent to thoseskilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are notlimitative of the present invention, and wherein:

FIG. 1 is a diagram illustrating a schematic arrangement of a failuredetection apparatus for an internal combustion engine according to thepresent invention;

FIG. 2 is a flowchart Illustrating a control routine for air flow sensor(AFS) failure determination according to a first embodiment of thepresent invention;

FIG. 3 is a flowchart illustrating another control routine for AFSfailure determination according to a second embodiment of the presentinvention;

FIG. 4 is a flowchart illustrating still another control routine for AFSfailure determination according to a third embodiment of the presentinvention;

FIG. 5 is a flowchart illustrating yet another control routine for AFSfailure determination according to a fourth embodiment of the presentinvention;

FIG. 6 is a flowchart illustrating a further control routine for AFSfailure determination according to a fifth embodiment of the presentinvention; and

FIG. 7 is a flowchart illustrating a still further control routine forAFS failure determination according to a sixth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be hereinafter described withreference to the accompanying drawings.

FIG. 1 illustrates a schematic arrangement of a failure detectionapparatus for an internal combustion engine according to the presentinvention. The arrangement of the failure detection apparatus of thepresent invention will be described first with reference to FIG. 1.

As shown in FIG. 1, an internal combustion engine 1 is a common-railin-line four-cylinder diesel engine, for example. In the common-railengine 1, a solenoid-operated fuel injection nozzle 4 is provided foreach cylinder so as to face a corresponding combustion chamber 2 and isconnected through a high-pressure pipe 5 to a common rail 6. The commonrail 6 is connected through a high-pressure pipe 7 a to a high-pressurepump 8, which is connected to a fuel tank 9 through a low-pressure pipe7 b. Since the engine 1 is a diesel engine, light oil is used as fuel.

A solenoid-operated intake throttle valve (exhaust flow rate adjustingmeans) 12 is arranged in an intake passage 10 of the engine 1. On anupstream side of the intake throttle valve 12 is provided an air flowsensor (AFS; fresh air quantity detecting means) 14 for outputting asignal S_(afs) based on which a fresh air quantity Q_(a) is detected.The intake throttle valve 12 comprises a butterfly valve, for example.Also, in the illustrated engine, a Karman vortex air flow sensor is usedas the air flow sensor 14 by way of example, but a hot-wire air flowsensor or the like may be used instead.

An after-treatment device 24 is inserted in an exhaust passage 20. Theafter-treatment device 24 comprises, for example, a continuouslyregenerating DPF (diesel particulate filter) including a DPF 24 b and anoxidation catalyst 24 a arranged on an upstream side of the DPF 24 b.

In the continuously regenerating DPF, an oxidizer (NO₂) is produced inthe oxidation catalyst 24 a and is used to continuously removeparticulate matter (PM) deposited on the downstream-side DPF 24 b byoxidation under a relatively high exhaust gas temperature condition, tothereby regenerate the DPF 24 b.

A λ sensor (O₂ sensor etc.; exhaust concentration detecting means) 26 isarranged at the exhaust passage 20 at a location upstream theafter-treatment device 24, to detect an excess air ratio λ of theexhaust system through detection of the oxygen concentration of exhaustgas. Instead of detecting the excess air ratio λ, the air-fuel ratio maybe detected, and in this case, an air-fuel ratio sensor (LAFS etc.) isused in place of the λ sensor 26.

The exhaust passage 20 is also provided with a solenoid-operated exhaustthrottle valve (exhaust flow rate adjusting means) 22. The exhaustthrottle valve 22 comprises a butterfly valve, for example, like theintake throttle valve 12, and is operated singly or together with theintake throttle valve 12 to adjust the exhaust flow rate, that is, theexhaust flow velocity, and thereby control the temperature of theexhaust gas in the exhaust passage 20 so that purification of theexhaust gas can be promoted at the cold-start of the engine 1, forexample. The exhaust throttle valve 22 also functions as an exhaustbrake.

An EGR passage 30 extends from a portion of the exhaust passage 20 nearthe engine 1, to allow part of the exhaust gas to be recirculated to theintake system as EGR gas. The EGR passage 30 is connected at the otherend to a portion of the intake passage 10 located downstream the intakethrottle valve 12. A solenoid-operated EGR valve 32 of which the openingcan be adjusted to a desired opening is inserted in the EGR passage 30.

An electronic control unit (ECU) 40 is connected at an input sidethereof with various sensors which include an accelerator positionsensor (APS) 44 for detecting the stroke of an accelerator pedal 42,that is, accelerator position θ_(acc), and a crank angle sensor 46 fordetecting an engine speed N_(e) through detection of crank angle,besides the air flow sensor 14 and the λ sensor 26.

The output side of the ECU 40 is connected with various devicesincluding a failure lamp 50 for indicating a variety of failurestatuses, in addition to the fuel injection nozzles 4, intake andexhaust throttle valves 12 and 22, and EGR valve 32.

Based on information input from the various sensors, operations of thevarious devices are controlled to properly control the operation of theengine 1. For example, based on the information from the acceleratorposition sensor 44, air flow sensor 14 and λ sensor 26, a fuel injectionquantity Q_(f) as well as the opening of the intake throttle valve 12are adjusted to control the operation of the engine 1, whereby not onlythe control of normal engine operation but the regeneration control ofthe after-treatment device 24, the opening control (exhaust flow ratecontrol means) of the intake and exhaust throttle valves 12 and 22 forpromoting exhaust gas purification, and the opening control (EGR valvecontrol means) of the EGR valve 32 are carried out.

Operation of the failure detection apparatus for the engine configuredas above will be now described.

A first embodiment will be explained first.

FIG. 2 is a flowchart illustrating a control routine for air flow sensor(AFS) failure determination, executed in the failure detection apparatusaccording to the first embodiment of the present invention. The controlroutine will be described with reference to the flowchart.

First, in Step S10, it is determined whether or not exhaust flow ratecontrol is under execution, that is, whether or not the exhaust flowrate is being adjusted by controlling the opening of one or both of theintake and exhaust throttle valves 12 and 22 toward the closed position.For example, just after the cold-start of the engine 1, the exhaust gaspurifying capability is low, and it is determined in this step whetheror not the exhaust flow rate control is executed in such situations topromote exhaust gas purification. If the decision in this step isaffirmative (Yes) and thus the exhaust flow rate control is underexecution, the routine proceeds to Step S12.

In Step S12 is set a target throttle valve opening for the intake andexhaust throttle valves 12 and 22 as a whole. In this instance, thetarget throttle valve opening (target adjustment amount, target valveopening) is set (target adjustment setting means) in accordance with thetemperature (cooling water temperature etc.) of the engine 1, forexample. Usually, a target value (predetermined value) λ₁ for the excessair ratio λ is set in accordance with the operating state of the engine1, and as the openings of the intake and exhaust throttle valves 12 and22 change, the exhaust gas or EGR gas is recirculated into thecombustion chamber 2 due to increase in the exhaust pressure, with theresult that the excess air ratio λ varies depending on the EGR quantity.Accordingly, the fuel injection quantity Q_(f) also is controlled inaccordance with the target throttle valve opening so that the excess airratio λ may be kept at the target value λ₁. In other words, the targetthrottle valve opening is set while controlling the excess air ratio λto the target value λ₁. In practice, a map showing the relationship ofthe target throttle valve opening with the engine speed N_(e), fuelinjection quantity Q_(f) and target value λ₁ is prepared beforehand byexperiment, and as soon as the target throttle valve opening is set, anappropriate fuel injection quantity Q_(f) is read from the map.

In Step S14, the target throttle valve opening set in the aforementionedmanner is corrected based on an actual excess air ratio λ detected bythe λ sensor 26. The target throttle valve opening is merely a commandvalue from the ECU 40 and is not an actual value. Thus, even if theoverall opening of the intake and exhaust throttle valves 12 and 22 iscontrolled so as to coincide with the target throttle valve openingcorresponding to the target value λ₁, a difference can occasionallyarise between the actual overall opening of the intake and exhaustthrottle valves. 12 and 22 and the target throttle valve opening. Suchan opening difference leads to an increased or decreased air quantity,causing a similar difference between the target value λ₁ and the actualexcess air ratio λ. Accordingly, the target value λ₁ is compared withthe actual excess air ratio λ, and based on the result of comparison,the target throttle valve opening is corrected so as to match the actualopening.

Specifically, a difference (absolute value) |λ−λ₁| between the targetvalue λ₁ and the actual excess air ratio λ detected by the λ sensor 26is obtained, and the target throttle valve opening is corrected by anamount corresponding to the difference. The corrected target throttlevalve opening obtained in this manner may be stored as a learned value.

Consequently, the target throttle valve opening is corrected to a propervalue matching the actual overall opening of the intake and exhaustthrottle valves 12 and 22.

In Step S16, a reference value for the fresh air quantity Q_(a), thatis, a fresh air quantity reference value, is set (fresh air quantityreference value setting means) in accordance with the appropriatelycorrected target throttle valve opening obtained as described above.Basically, the reference value for the fresh air quantity Q_(a), or thefresh air quantity reference value, is set in accordance with theoperating state (engine speed N_(e), fuel injection quantity Q_(f),engine torque, throttle valve opening (throttle opening), manifold airpressure, etc.) of the engine 1. In this step, the fresh air quantityreference value thus set is corrected by using the target throttle valveopening. In practice, a map showing the relationship between fresh airquantity Q_(a)′ applicable during the exhaust flow rate control and thetarget throttle valve opening is prepared beforehand, and an appropriatefresh air quantity Q_(a)′ read from the map is set as the fresh airquantity reference value.

Since the target throttle valve opening has been corrected to a propervalue matching the actual overall opening of the intake and exhaustthrottle valves 12 and 22, as mentioned above, the fresh air quantityreference value can be set to a highly accurate value as in the casewhere the exhaust flow rate is not controlled through the adjustment ofthe intake and exhaust throttle valves 12 and 22.

If the decision in Step S10 is negative (No) and it is judged that theexhaust flow rate control is not under execution, the routine proceedsto Step S20.

In this case, a fresh air quantity Q_(a), normally obtained inaccordance with the operating state of the engine 1 is set directly asthe fresh air quantity reference value, without regard to the targetthrottle valve opening.

In Step S22, a difference (absolute value) |S_(afs)−reference value|between the output signal S_(afs) of the air flow sensor 14 and thefresh air quantity reference value is derived as X (|S_(afs)−referencevalue|=X). Namely, if the air flow sensor 14 is functioning normally,its output signal S_(afs) should coincide with the fresh air quantityreference value. In the event the output signal S_(afs) differs from thefresh air quantity reference value, the difference is derived as X inthis step.

Then, in Step S24, it is determined whether or not the difference X hasa value larger than or equal to a predetermined value X₁ (very smallvalue) (X≧X₁).

If the decision in Step S24 is affirmative (Yes) and it is judged thatthe difference X is larger than or equal to the predetermined value X₁,it can be concluded that the air flow sensor 14 is not functioningnormally and is in an abnormal state and that failure of the air flowsensor 14 has occurred (failure detecting means). In this case,therefore, it is ascertained in Step S26 that the difference X remainslarger than or equal to the predetermined value X₁ for a predeterminedtime t₁, and in Step S28, the failure lamp 50 is turned on to notify thedriver of the failure of the air flow sensor 14. Also, in Step S30, afailure code corresponding to the failure of the air flow sensor 14 isrecorded in a memory within the ECU 40.

Since the fresh air quantity reference value, in particular, is set withhigh accuracy based on the proper target throttle valve opening just asin the case where the intake and exhaust throttle valves 12 and 22 arenot operated, failure of the air flow sensor 14 can be detected withprecision, regardless of the adjustment of the exhaust flow rate, andthe reliability of the air flow sensor 14 can be improved. Thus, incases where the output information from the air flow sensor 14 is usedfor controlling the regeneration of the after-treatment device 24, theregeneration can be controlled optimally, making it possible to furtherimprove the exhaust gas characteristics.

If the decision in Step S24 is negative (No) and it is judged that thedifference X is smaller than the predetermined value X₁ (very smallvalue), it can be concluded that the air flow sensor 14 is functioningnormally without failure, and thus the subsequent steps of the routineare not executed.

A second embodiment will be now described.

FIG. 3 is a flowchart illustrating a control routine for air flow sensor(AFS) failure determination, executed in the failure detection apparatusaccording to the second embodiment of the present invention. In thefollowing description of the flowchart, only the differences between thefirst and second embodiments will be explained.

In the second embodiment, after the target throttle valve opening is setin Step S12, the fresh air quantity reference value is set immediatelythereafter based on the target throttle valve opening in Step S16,without correcting the target throttle valve opening, unlike the firstembodiment.

Then, in Step S17, it is determined whether or not the actual excess airratio λ detected by the λ sensor 26 is equal to the target value λ₁(λ=λ₁). In other words, it is determined whether or not a differencebetween the target value λ₁ and the actual excess air ratio λ has beencaused due to an opening difference between the actual overall openingof the intake and exhaust throttle valves 12 and 22 and the targetthrottle valve opening.

If the decision in Step S17 is affirmative (Yes) and it is judged thatthe actual excess air ratio λ and the target value λ₁ are equal to eachother, it can be concluded that the target throttle valve opening hasbeen set to a proper value matching the actual overall opening of theintake and exhaust throttle valves 12 and 22, and accordingly, theroutine proceeds to Step S22.

On the other hand, if the decision in Step S17 is negative (No) and itis judged that the actual excess air ratio λ and the target value λ₁differ from each other, the overall opening of the intake and exhaustthrottle valves 12 and 22 is corrected, in Step S18, such that theactual excess air ratio λ becomes equal to the target value λ₁.

Namely, in the first embodiment, the target throttle valve opening iscorrected so as to match the actual overall opening of the intake andexhaust throttle valves 12 and 22. In the second embodiment, the actualoverall opening of the intake and exhaust throttle valves 12 and 22 iscorrected so as to match the target throttle valve opening.

Thus, the actual overall opening of the intake and exhaust throttlevalves 12 and 22 is corrected to a proper value matching the targetthrottle valve opening, and accordingly, the fresh air quantityreference value can be set to a highly accurate value as in the casewhere the intake and exhaust throttle valves 12 and 22 are not operated.

Consequently, failure of the air flow sensor 14 can be detected withprecision, regardless of the adjustment of the exhaust flow rate, andthe reliability of the air flow sensor 14 can be improved. In caseswhere the information from the air flow sensor 14 is used forcontrolling the regeneration of the after-treatment device 24,therefore, the regeneration can be controlled optimally, making itpossible to further improve the exhaust gas characteristics.

A third embodiment will be now described.

FIG. 4 is a flowchart illustrating a control routine for air flow sensor(AFS) failure-determination, executed in the failure detection apparatusaccording to the third embodiment of the present invention. In thefollowing description of the flowchart, only the differences between thethird embodiment and the first or second embodiment will be explained.

In the third embodiment, after the target throttle valve opening is setin Step S12, the fresh air quantity reference value is set immediatelythereafter based on the target throttle valve opening in Step S16, likethe second embodiment.

Then, in Step S17, it is determined whether or not the actual excess airratio λ detected by the λ sensor 26 is equal to the target value λ₁(λ=λ₁), as in the second embodiment.

If the decision in Step S17 is affirmative (Yes) and it is judged thatthe actual excess air ratio λ and the target value λ₁ are equal to eachother, it can be concluded that the target throttle valve opening hasbeen set to a proper value matching the actual overall opening of theintake and exhaust throttle valves 12 and 22, and accordingly, theroutine proceeds to Step S22.

On the other hand, if the decision in Step S17 is negative (No) and itis judged that the actual excess air ratio λ and the target value λ₁differ from each other, the exhaust flow rate control is suspended inStep S19, then a normally obtained fresh air quantity Q_(a) is directlyset as the fresh air quantity reference value in Step S20, and theroutine proceeds to Step S22.

Namely, in the third embodiment, if there is a difference between theactual excess air ratio λ and the target value λ₁, it is concluded thatthe fresh air quantity reference value cannot be set with accuracy, andthe exhaust flow rate control itself is suspended. Thus, using the freshair quantity Q_(a) obtained normally in accordance with the operatingstate of the engine 1 as the fresh air quantity reference value, the airflow sensor 14 is diagnosed, without adjusting the exhaust flow rate.

In this case, the diagnosis of the air flow sensor 14 is not suspendedbut is continuously performed even while the exhaust flow rate controlis suspended, whereby the occasion for diagnosis is not reduced.

Consequently, the fresh air quantity reference value can always be setto an accurate value, regardless of the degree to which the exhaust flowrate is adjusted. Thus, failure of the air flow sensor 14 can bedetected with precision, making it possible to improve the reliabilityof the air flow sensor 14.

Fourth to sixth embodiments will be now described.

In the fourth to sixth embodiments, failure of the air flow sensor isdetermined taking account of not only the exhaust flow rate control, asin the first to third embodiments described above, but also EGR control.The fourth to sixth embodiments correspond to the first to thirdembodiments, respectively.

FIG. 5 is a flowchart illustrating a control routine for air flow sensor(AFS) failure determination, executed in the failure detection apparatusaccording to the fourth embodiment of the present invention. In thefollowing description of the flowchart, only the differences between thefourth and first embodiments will be explained.

First, in Step S10, it is determined whether or not the exhaust flowrate control is under execution, that is, whether or not the exhaustflow rate is being adjusted by controlling the opening of one or both ofthe intake and exhaust throttle valves 12 and 22 toward the closedposition, as in the foregoing embodiments. If the decision in this stepis affirmative (Yes) and thus the exhaust flow rate control is underexecution, the routine proceeds to Step S11.

In Step S11, it is determined whether or not the EGR is under execution,that is, whether or not the EGR valve 32 is opened to introduce the EGRgas into the intake system while the exhaust flow rate control isperformed. If the decision in this step is affirmative (Yes) and thusthe EGR is under execution, the routine proceeds to Step S12′.

In Step S12′ are set a target throttle valve opening for the intake andexhaust throttle valves 12 and 22 as a whole and a target EGR valveopening for the EGR valve 32. In this instance, the target throttlevalve opening (target adjustment amount) is set (target adjustmentsetting means) in accordance with the temperature (cooling watertemperature etc.) of the engine 1, as mentioned above, and the targetEGR valve opening is set (target opening setting means) in accordancewith the engine speed N_(e) and the fuel injection quantity Q_(f).

Usually, the target value (predetermined value) λ₁ for the excess airratio λ is set in accordance with the operating state of the engine 1,and as the openings of the intake and exhaust throttle valves 12 and 22change, the exhaust gas or EGR gas is recirculated into the combustionchamber 2, as mentioned above, with the result that the excess air ratioλ varies depending on the EGR quantity. Accordingly, the fuel injectionquantity Q_(f) also is controlled in accordance with the target throttlevalve opening so that the excess air ratio λ may be kept at the targetvalue λ₁. In other words, the target throttle valve opening is set whilecontrolling the excess air ratio λ to the target value λ₁. In practice,the fuel injection quantity Q_(f) is read from the map preparedbeforehand, as mentioned above.

Also, as the target value λ₁, changes, the quantity of the EGR gasintroduced, that is, the target EGR valve opening, also changes withrelation to the opening of the intake throttle valve 12 and the fuelinjection quantity Q_(f). Accordingly, the target EGR valve opening isset also based on the target value λ₁ for the excess air ratio λ. Inpractice, a map showing the relationship of the target EGR valve openingwith the engine speed N_(e), fuel injection quantity Q_(f) and targetvalue λ₁ is prepared beforehand by experiment, and an appropriate targetEGR valve opening is read from the map.

In Step S14′, the target throttle valve opening and target EGR valveopening set in the aforementioned manner are corrected based on anactual excess air ratio λ detected by the λ sensor 26. The targetthrottle valve opening and the target EGR valve opening are merelycommand values from the ECU 40 and are not actual values. Thus, even ifthe overall opening of the intake and exhaust throttle valves 12 and 22is controlled so as to coincide with the target throttle valve openingcorresponding to the target value λ₁ and also the opening of the EGRvalve 32 is controlled so as to coincide with the target EGR valveopening corresponding to the target value λ₁, a difference canoccasionally arise between the actual overall opening of the intake andexhaust throttle valves 12 and 22 and the target throttle valve openingor between the actual opening of the EGR valve 32 and the target EGRvalve opening. Such an opening difference causes a similar differencebetween the target value λ₁ and the actual excess air ratio λ.Accordingly, the target value λ₁ is compared with the actual excess airratio λ, and based on the result of comparison, the target throttlevalve opening and the target EGR valve opening are corrected so as tomatch their actual openings.

Specifically, a difference (absolute value) |λ−λ₁| between the targetvalue λ₁ and the actual excess air ratio λ detected by the λ sensor 26is obtained, and the target throttle valve opening and the target EGRvalve opening are corrected by an amount corresponding to thedifference. In this case, the target throttle valve opening and thetarget EGR valve opening may be corrected as a whole based on thedifference |λ−λ₁|.

In Step S16′, a reference value for the fresh air quantity Q_(a), thatis, the fresh air quantity reference value, is set (fresh air quantityreference value setting means) in accordance with the target throttlevalve opening and target EGR valve opening obtained as described above.Basically, the reference value for the fresh air quantity Q_(a), or thefresh air quantity reference value, is set in accordance with theoperating state (engine speed N_(e), fuel injection quantity Q_(f),engine torque, throttle opening, manifold air pressure, etc.) of theengine 1. In this step, the fresh air quantity reference value thus setis corrected by using the target throttle valve opening and the targetEGR valve opening. In practice, a map showing the relationship betweenfresh air quantity Q_(a)′ applicable during the exhaust flow ratecontrol and the target throttle valve opening is prepared beforehand, adifference (Q_(a)′−Q_(egr)) between the fresh air quantity Q_(a)′ readfrom the map and an EGR gas quantity Q_(egr) corresponding to the targetEGR valve opening is obtained, and a reference value corresponding tothe difference (Q_(a)′−Q_(egr)) is set as the fresh air quantityreference value. Alternatively, the reference value for the fresh airquantity Q_(a)′ not including the EGR gas may be corrected by using avalue corresponding to the target EGR valve opening.

Since the target throttle valve opening and the target EGR valve openinghave been corrected as a whole to proper values matching the actualopenings of the intake and exhaust throttle valves 12 and 22 and EGRvalve 32, as mentioned above, the fresh air quantity reference value canbe set to a highly accurate value as in the case where the exhaust flowrate is not adjusted or the EGR gas is not introduced.

If the decision in Step S10 is negative (No) and it is judged that theexhaust flow rate control is not under execution, or if the decision inStep S11 is negative (No) and it is judged that no EGR gas is beingintroduced into the intake system, the routine proceeds to Step S20.

In this case, a fresh air quantity Q_(a) normally obtained in accordancewith the operating state of the engine 1 is set directly as the freshair quantity reference value, without regard to the target throttlevalve opening or the target EGR valve opening.

Then, in Step S22, a difference (absolute value) |S_(afs)−referencevalue| between the output signal S_(afs) of the air flow sensor 14 andthe fresh air quantity reference value is derived as X(|S_(afs)−reference value|=X), like the foregoing embodiments.Subsequently, in Step S24, it is determined whether or not thedifference X has a value larger than or equal to the predetermined valueX₁ (very small value) (X≧X₁). If the decision in this step isaffirmative (Yes) and it is judged that the difference X is larger thanor equal to the predetermined value X₁, it is concluded that failure ofthe air flow sensor 14 has occurred (failure detecting means), and thefailure lamp 50 is turned on, in Step S28, to notify the driver of thefailure of the air flow sensor 14. Also, in Step S30, the failure codecorresponding to the failure of the air flow sensor 14 is recorded inmemory within the ECU 40.

Thus, even while the exhaust flow rate control is performed and at thesame time the EGR gas is introduced into the intake system, failure ofthe air flow sensor 14 can be detected with high accuracy, as in thefirst to third embodiments described above, whereby the reliability ofthe air flow sensor 14 can be improved.

FIG. 6 is a flowchart illustrating a control routine for air flow sensor(AFS) failure determination, executed in the failure detection apparatusaccording to the fifth embodiment of the present invention. In thefollowing description of the flowchart, only the differences between thefourth and fifth embodiments will be explained.

In the fifth embodiment, after the target throttle valve opening and thetarget EGR valve opening are set in Step S12′, the fresh air quantityreference value is set immediately thereafter based on the targetthrottle valve opening and the target EGR valve opening, in Step S16′,without correcting the target openings, unlike the fourth embodiment.

Then, in Step S17, it is determined whether or not the actual excess airratio λ detected by the λ sensor 26 is equal to the target value λ₁(λ=λ₁). If the decision in this step is affirmative (Yes) and it isjudged that the actual excess air ratio λ and the target value λ₁ areequal to each other, it can be concluded that the target throttle valveopening and the target EGR valve opening have been set to proper valuesmatching their actual openings, and accordingly, the routine proceeds toStep S22.

On the other hand, if the decision in Step S17 is negative (No) and itis judged that the actual excess air ratio λ and the target value λ₁differ from each other, the overall opening of the intake and exhaustthrottle valves 12 and 22 and the opening of the EGR valve 32 arecorrected, in Step S18′, such that the actual excess air ratio λ becomesequal to the target value λ₁.

Namely, in the fourth embodiment, the target throttle valve opening andthe target EGR valve opening are corrected so as to match their actualopenings. In the fifth embodiment, the actual overall opening of theintake and exhaust throttle valves 12 and 22 and the actual opening ofthe EGR valve 32 are corrected so as to match their target openings.

Thus, the actual overall opening of the intake and exhaust throttlevalves 12 and 22 and the actual opening of the EGR valve 32 arecorrected to proper values matching their target openings, andaccordingly, the fresh air quantity reference value can be set to ahighly accurate value as in the case where the exhaust flow rate is notadjusted or the EGR gas is not introduced.

Consequently, failure of the air flow sensor 14 can be detected withprecision, regardless of the exhaust flow rate adjustment or the EGR gasintroduction, so that the reliability of the air flow sensor 14 can beimproved.

FIG. 7 is a flowchart illustrating a control routine for air flow sensor(AFS) failure determination, executed in the failure detection apparatusaccording to the sixth embodiment of the present invention. In thefollowing description of the flowchart, only the differences between thesixth embodiment and the fourth or fifth embodiment will be explained.

In the sixth embodiment, after the target throttle valve opening and thetarget EGR valve opening are set in Step S12′, the fresh air quantityreference value is set immediately thereafter based on the targetthrottle valve opening and the target EGR valve opening in Step S16′,like the fifth embodiment.

Then, in Step S17, it is determined whether or not the actual excess airratio λ detected by the λ sensor 26 is equal to the target value λ₁(λ−λ₁). If the decision in this step is affirmative (Yes) and it isjudged that the actual excess air ratio λ and the target value λ₁ areequal to each other, it can be concluded that the target throttle valveopening and the target EGR valve opening have been set to proper valuesmatching their actual openings, and accordingly, the routine proceeds toStep S22.

On the other hand, if the decision in Step S17 is negative (No) and itis judged that the actual excess air ratio λ and the target value λ₁differ from each other, the exhaust flow rate control and the EGRcontrol are suspended in Step S19′, then a normally obtained fresh airquantity Q_(a) is directly set as the fresh air quantity reference valuein Step S20, and the routine proceeds to Step S22.

Namely, in the sixth embodiment, if there is a difference between theactual excess air ratio λ and the target value λ₁, it is concluded thatthe fresh air quantity reference value cannot be set with accuracy, andthe exhaust flow rate control and the EGR control are suspended. Thus,using the fresh air quantity Q_(a) obtained normally in accordance withthe operating state of the engine 1 as the fresh air quantity referencevalue, the air flow sensor 14 is diagnosed, without adjusting theexhaust flow rate or introducing the EGR gas.

This permits the fresh air quantity reference value to be always set toan accurate value, without reducing the occasion for diagnosis andregardless of the degree to which the exhaust flow rate is adjusted orto which the EGR gas is introduced. Also in this case, therefore,failure of the air flow sensor 14 can be detected with precision, makingit possible to improve the reliability of the air flow sensor 14.

While the preferred embodiments of the present invention have beendescribed, it should be noted that the present invention is not limitedto the foregoing embodiments alone.

For example, in the foregoing embodiments, the λ sensor (O₂ sensor etc.)26 is used as the exhaust concentration detecting means and the excessair ratio λ or air-fuel ratio of the exhaust system, which is detectedby the λ sensor 26, is compared with the target value (predeterminedvalue) λ₁. The present invention is, however, not limited to suchconfiguration and may be provided with a means for detecting theconcentration of gas flowing into the combustion chamber 2 so that thedetected gas concentration may be compared with a predetermined value.

Also, in the foregoing embodiments, both the intake throttle valve 12and the exhaust throttle valve 22 are provided as the exhaust flow rateadjusting means, but the exhaust flow rate adjusting means may beconstituted by either one of the intake and exhaust throttle valves 12and 22.

Further, although in the foregoing embodiments, a diesel engine is usedas the engine 1, the engine 1 may alternatively be a gasoline engine.

1. A failure detection apparatus for an internal combustion engine,comprising: fresh air quantity detecting means arranged in an intakesystem of the engine, for detecting a quantity of fresh air introducedinto a combustion chamber of the engine; fresh air quantity referencevalue setting means for setting a reference value for the fresh airquantity in accordance with an operating state of the engine; failuredetecting means for detecting abnormality of said fresh air quantitydetecting means, based on a result of comparison between the fresh airquantity detected by said fresh air quantity detecting means and thereference value set by said fresh air quantity reference value settingmeans; exhaust flow rate adjusting means arranged in at least one of theintake system and exhaust system of the engine, for adjusting an exhaustflow rate; target adjustment setting means for setting a targetadjustment amount for said exhaust flow rate adjusting means inaccordance with the operating state of the engine such that an air-fuelratio or excess air ratio of the exhaust system becomes equal to apredetermined value corresponding to the operating state; and exhaustflow rate control means for controlling said exhaust flow rate adjustingmeans in accordance with the target adjustment amount set by said targetadjustment setting means, wherein said fresh air quantity referencevalue setting means sets the reference value in accordance with not onlythe operating state of the engine but also the target adjustment amountset for said exhaust flow rate adjusting means by said target adjustmentsetting means.
 2. The failure detection apparatus according to claim 1,wherein said exhaust flow rate adjusting means includes at least one ofan intake throttle valve arranged in the intake system for adjusting thefresh air quantity and an exhaust throttle valve arranged in the exhaustsystem for directly adjusting the exhaust flow rate, and said fresh airquantity reference value setting means sets the reference value inaccordance with not only the operating state of the engine but also atarget valve opening set for said at least one of the intake and exhaustthrottle valves by said target adjustment setting means.
 3. The failuredetection apparatus according to claim 1, further comprising exhaustconcentration detecting means for detecting an exhaust concentration tothereby detect the air-fuel ratio or excess air ratio of the exhaustsystem, wherein said fresh air quantity reference value setting meanscorrects the target adjustment amount for said exhaust flow rateadjusting means, based on a difference between the air-fuel ratio orexcess air ratio of the exhaust system, detected by said exhaustconcentration detecting means, and the predetermined value, and sets thereference value based on the corrected target adjustment amount.
 4. Thefailure detection apparatus according to claim 1, further comprisingexhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, wherein said exhaust flow rate control meanscorrects an amount of adjustment by said exhaust flow rate adjustingmeans such that the air-fuel ratio or excess air ratio of the exhaustsystem, detected by said exhaust concentration detecting means,coincides with the predetermined value.
 5. The failure detectionapparatus according to claim 1, further comprising exhaust concentrationdetecting means for detecting an exhaust concentration to thereby detectthe air-fuel ratio or excess air ratio of the exhaust system, wherein,when the air-fuel ratio or excess air ratio of the exhaust system,detected by said exhaust concentration detecting means, is differentfrom the predetermined value, said fresh air quantity reference valuesetting means suspends the control of said exhaust flow rate adjustingmeans by said exhaust flow rate control means and sets the referencevalue based solely on the operating state of the engine.
 6. A failuredetection apparatus for an internal combustion engine, comprising: freshair quantity detecting: means arranged in an intake system of theengine, for detecting a quantity of fresh air introduced into acombustion chamber of the engine; fresh air quantity reference valuesetting means for setting a reference value for the fresh air quantityin accordance with an operating state of the engine; failure detectingmeans for detecting abnormality of said fresh air quantity detectingmeans, based on a result of comparison between the fresh air quantitydetected by said fresh air quantity detecting means and the referencevalue set by said fresh air quantity reference value setting means;exhaust flow rate adjusting means arranged in one of the intake systemand exhaust system of the engine, for adjusting an exhaust flow rate;target adjustment setting means for setting a target adjustment amountfor said exhaust flow rate adjusting means in accordance with theoperating state of the engine such that an air-fuel ratio or excess airratio of the exhaust system becomes equal to a predetermined valuecorresponding to the operating state; exhaust flow rate control meansfor controlling said exhaust flow rate adjusting means in accordancewith the target adjustment amount set by said target adjustment settingmeans; an EGR passage for allowing part of exhaust gas to berecirculated from the exhaust system of the engine to the intake systemas EGR gas; an EGR valve inserted in said EGR passage, for controlling aquantity of the EGR gas by varying an opening thereof; target openingsetting means for setting a target EGR valve opening for said EGR valvein accordance with the operating state of the engine such that theair-fuel ratio or excess air ratio of the exhaust system becomes equalto the predetermined value; and EGR valve control means for controllingsaid EGR valve in accordance with the target EGR valve opening set bysaid target opening setting means, wherein said fresh air quantityreference value setting means sets the reference value in accordancewith not only the operating state of the engine but also the targetadjustment amount set for said exhaust flow rate adjusting means by saidtarget adjustment setting means and the target EGR valve opening set bysaid target opening setting means.
 7. The failure detection apparatusaccording to claim 6, wherein said exhaust flow rate adjusting meansincludes at least one of an intake throttle valve arranged in the intakesystem for adjusting the fresh air quantity and an exhaust throttlevalve arranged in the exhaust system for directly adjusting the exhaustflow rate, and said fresh air quantity reference value setting meanssets the reference value in accordance with not only the operating stateof the engine but also a target valve opening set for said at least oneof the intake and exhaust throttle valves by said target adjustmentsetting means and the target EGR valve opening set by said targetopening setting means.
 8. The failure detection apparatus according toclaim 6, further comprising exhaust concentration detecting means fordetecting an exhaust concentration to thereby detect the air-fuel ratioor excess air ratio of the exhaust system, wherein said fresh airquantity reference value setting means corrects at least one of thetarget adjustment amount and the target EGR valve opening based on adifference between the air-fuel ratio or excess air ratio of the exhaustsystem, detected by said exhaust concentration detecting means, and thepredetermined value, and sets the reference value based on said at leastone of the corrected target adjustment amount and the corrected targetEGR valve opening.
 9. The failure detection apparatus according to claim6, further comprising exhaust concentration detecting means fordetecting an exhaust concentration to thereby detect the air-fuel ratioor excess air ratio of the exhaust system, wherein said exhaust flowrate control means corrects an amount of adjustment by said exhaust flowrate adjusting means such that the air-fuel ratio or excess air ratio ofthe exhaust system, detected by said exhaust concentration detectingmeans, coincides with the predetermined value, and said EGR valvecontrol means corrects the opening of said EGR valve such that theair-fuel ratio or excess air ratio of the exhaust system, detected bysaid exhaust concentration detecting means, coincides with thepredetermined value.
 10. The failure detection apparatus according toclaim 6, further comprising exhaust concentration detecting means fordetecting an exhaust concentration to thereby detect the air-fuel ratioor excess air ratio of the exhaust system, wherein, when the air-fuelratio or excess air ratio of the exhaust system, detected by saidexhaust concentration detecting means, is different from thepredetermined value, said fresh air quantity reference value settingmeans suspends the control of said exhaust flow rate adjusting means bysaid exhaust flow rate control means as well as the control of said EGRvalve by said EGR valve control means and sets the reference value basedsolely on the operating state of the engine.
 11. A failure detectionmethod implemented by a failure detection apparatus for an internalcombustion engine, the failure detection apparatus including fresh airquantity detecting means for detecting a quantity of fresh airintroduced into a combustion chamber of the engine and exhaust flow rateadjusting means for adjusting an exhaust flow rate in at least one ofintake and exhaust systems of the engine, said failure detection methodcomprising: a target adjustment setting step of setting a targetadjustment amount for the exhaust flow rate adjusting means inaccordance with an operating state of the engine such that an air-fuelratio or excess air ratio of the exhaust system becomes equal to apredetermined value corresponding to the operating state; an exhaustflow rate control step of controlling the exhaust flow rate adjustingmeans in accordance with the target adjustment amount set in said targetadjustment setting step; a fresh air quantity reference value settingstep of setting a reference value for the fresh air quantity inaccordance with the operating state of the engine and the targetadjustment amount set for the exhaust flow rate adjusting means in saidtarget adjustment setting step; and a failure detecting step ofdetecting abnormality of the fresh air quantity detecting means, basedon a result of comparison between the fresh air quantity detected by thefresh air quantity detecting means and the reference value set in saidfresh air quantity reference value setting step.
 12. The failuredetection method according to claim 11, wherein said exhaust flow rateadjusting means includes at least one of an intake throttle valvearranged in the intake system for adjusting the fresh air quantity andan exhaust throttle valve arranged in the exhaust system for directlyadjusting the exhaust flow rate, and said fresh air quantity referencevalue setting step comprises setting the reference value in accordancewith not only the operating state of the engine but also a target valveopening set for said at least one of the intake and exhaust throttlevalves in said target adjustment setting step.
 13. The failure detectionmethod according to claim 11, wherein said failure detection apparatusfurther includes exhaust concentration detecting means for detecting anexhaust concentration to thereby detect the air-fuel ratio or excess airratio of the exhaust system, and said fresh air quantity reference valuesetting step comprises correcting the target adjustment amount for theexhaust flow rate adjusting means based on a difference between theair-fuel ratio or excess air ratio of the exhaust system, detected bythe exhaust concentration detecting means, and the predetermined value,and setting the reference value based on the corrected target adjustmentamount.
 14. The failure detection method according to claim 11, whereinsaid failure detection apparatus further includes exhaust concentrationdetecting means for detecting an exhaust concentration to thereby detectthe air-fuel ratio or excess air ratio of the exhaust system, and saidexhaust flow rate control step comprises correcting an amount ofadjustment by the exhaust flow rate adjusting means such that theair-fuel ratio or excess air ratio of the exhaust system, detected bythe exhaust concentration detecting means, coincides with thepredetermined value.
 15. The failure detection method according to claim11, wherein said failure detection apparatus further includes exhaustconcentration detecting means for detecting an exhaust concentration tothereby detect the air-fuel ratio or excess air ratio of the exhaustsystem, and if the air-fuel ratio or excess air ratio of the exhaustsystem, detected by the exhaust concentration detecting means, isdifferent from the predetermined value, said fresh air quantityreference value setting step suspends the control of the exhaust flowrate adjusting means in said exhaust flow rate control step and sets thereference value based solely on the operating state of the engine.
 16. Afailure detection method implemented by a failure detection apparatusfor an internal combustion engine, the failure detection apparatusincluding fresh air quantity detecting means for detecting a quantity offresh air introduced into a combustion chamber of the engine, exhaustflow rate adjusting means for adjusting an exhaust flow rate in at leastone of intake and exhaust systems of the engine, an EGR passage forallowing part of exhaust gas to be recirculated from the exhaust systemof the engine to the intake system as EGR gas, and an EGR valve insertedin the EGR passage for controlling a quantity of the EGR gas by varyingan opening thereof, said failure detection method comprising: a targetadjustment setting step of setting a target adjustment amount for theexhaust flow rate adjusting means in accordance with an operating stateof the engine such that an air-fuel ratio or excess air ratio of theexhaust system becomes equal to a predetermined value corresponding tothe operating state; an exhaust flow rate control step of controllingthe exhaust flow rate adjusting means in accordance with the targetadjustment amount set in said target adjustment setting step; a targetopening setting step of setting a target EGR valve opening for the EGRvalve in accordance with the operating state of the engine such that theair-fuel ratio or excess air ratio of the exhaust system becomes equalto the predetermined value; an EGR valve control step of controlling theEGR valve in accordance with the target EGR valve opening set in saidtarget opening setting step; a fresh air quantity reference valuesetting step of setting a reference value for the fresh air quantity inaccordance with the operating state of the engine, the target adjustmentamount set for the exhaust flow rate adjusting means in said targetadjustment setting step, and the target EGR valve opening set in saidtarget opening setting step; and a failure detecting step of detectingabnormality of the fresh air quantity detecting means, based on a resultof comparison between the fresh air quantity detected by the fresh airquantity detecting means and the reference value set in said fresh airquantity reference value setting step.
 17. The failure detection methodaccording to claim 16, wherein the exhaust flow rate adjusting meansincludes at least one of an intake throttle valve arranged in the intakesystem for adjusting the fresh air quantity and an exhaust throttlevalve arranged in the exhaust system for directly adjusting the exhaustflow rate, and said fresh air quantity reference value setting stepcomprises setting the reference value in accordance with not only theoperating state of the engine but also a target valve opening set forsaid at least one of the intake and exhaust throttle valves in saidtarget adjustment setting step and the target EGR valve opening set insaid target opening setting step.
 18. The failure detection methodaccording to claim 16, wherein said failure detection apparatus furtherincludes exhaust concentration detecting means for detecting an exhaustconcentration to thereby detect the air-fuel ratio or excess air ratioof the exhaust system, and said fresh air quantity reference valuesetting step comprises correcting at least one of the target adjustmentamount and the target EGR valve opening based on a difference betweenthe air-fuel ratio or excess air ratio of the exhaust system, detectedby the exhaust concentration detecting means, and the predeterminedvalue, and setting the reference value based on said at least one of thecorrected target adjustment amount and the corrected target EGR valveopening.
 19. The failure detection method according to claim 16, whereinsaid failure detection apparatus further includes exhaust concentrationdetecting means for detecting an exhaust concentration to thereby detectthe air-fuel ratio or excess air ratio of the exhaust system, saidexhaust flow rate control step comprises correcting an amount ofadjustment by the exhaust flow rate adjusting means such that theair-fuel ratio or excess air ratio of the exhaust system, detected bythe exhaust concentration detecting means, coincides with thepredetermined value, and said EGR valve control step comprisescorrecting the opening of the EGR valve such that the air-fuel ratio orexcess air ratio of the exhaust system, detected by the exhaustconcentration detecting means, coincides with the predetermined value.20. The failure detection method according to claim 16, wherein saidfailure detection apparatus further includes exhaust concentrationdetecting means for detecting an exhaust concentration to thereby detectthe air-fuel ratio or excess air ratio of the exhaust system, and if theair-fuel ratio or excess air ratio of the exhaust system, detected bythe exhaust concentration detecting means, is different from thepredetermined value, said fresh air quantity reference value settingstep suspends the control of the exhaust flow rate adjusting means insaid exhaust flow rate control step as well as the control of the EGRvalve in said EGR valve control step and sets the reference value basedsolely on the operating state of the engine.